Rename structPointer_Map to structPointer_NewAt.

It was never map-specific, and will soon be used for other types.
diff --git a/proto/pointer_reflect.go b/proto/pointer_reflect.go
index c68b125..749919d 100644
--- a/proto/pointer_reflect.go
+++ b/proto/pointer_reflect.go
@@ -144,8 +144,8 @@
 	return structPointer_ifield(p, f).(*map[int32]Extension)
 }
 
-// Map returns the reflect.Value for the address of a map field in the struct.
-func structPointer_Map(p structPointer, f field, typ reflect.Type) reflect.Value {
+// NewAt returns the reflect.Value for a pointer to a field in the struct.
+func structPointer_NewAt(p structPointer, f field, typ reflect.Type) reflect.Value {
 	return structPointer_field(p, f).Addr()
 }